1. Field of the Invention
The present invention relates to a device for transmitting optical signals between units that are rotatable relative to each other. This is for simultaneously transmitting a plurality of optical signals along a plurality of channels. Preferably analog signals are transferred over an inventive rotary joint.
2. Description of the Related Art
Various transmission systems are known for transmitting digital optical signals between units that are rotatable relative to each other. These are referred to as rotary coupling or rotary joint.
An optical rotary joint for a plurality of channels, comprising a Dove prism, is disclosed in U.S. Pat. No. 5,568,578. Light is transferred from a first collimator to a second collimator, wherein a derotating prism is in the optical path between the collimators, the prism rotating with half the speed or angular displacement between the collimators. Due to mechanical and optical tolerances the attenuation changes with rotation. It is dependent of the angular position between the rotating and the stationary part of the rotary joint. Such a varying attenuation causes changing in the optical signal level. Accordingly analog signals, carrying the information in the amplitude, can hardly be transferred. This does not matter for digital signals, where only zero and one states have to be transferred.
EP 1476969 discloses a rotating data transmission device in which the light is transmitted by multiple reflections in mirror-coated trench. The attenuation of the incident light is varying significantly according to the angular position between the rotating and the stationary part.